In conventional, directed single lobe radar antennas the scanning of a target involves a risk of interception of the radiated radar signal, detection of said signal being dependent on the effective radiated power (ERP).
For reducing the risk of detection the radiated power has previously been reduced by distributing the power in the domains of time and frequency. However, this reduction of power has not been quite sufficient in practice to protect against detection.
British patent specification No. 1.302.923 discloses a scanning method for a fixed antenna where the lobe pattern of the antenna is varied by modulation of the antenna properties. The modulation is dependent on the radiation angle and is identified by means of correlators or matched filters. A control unit generates codes including modulation signals which represent phase and amplitude functions for each element of the antenna. The modulation signals are simultaneously supplied to a respective modulation member of each antenna element. The antenna transmits in sequence as many lobe patterns as the number of codes which are fed from the control unit to said modulation members.
In theory the modulation of the antenna properties according to said British patent specification should be satisfactory and would result in a small risk of detection due to a possible reduction of the radiated power. However, said method requires a large number of controllable modulation members, viz. equal to the number of antenna elements. The multitude of said controllable modulation members reduces the reliability, in spite of the fact that the antenna is fixed as is required for this method. Morever, the fact that the antenna is fixed and therefore covers a limited solid angle makes the method less attractive.
A more serious drawback of said method is that (as a result of the repeated variation of the lobe pattern which involves an undesired phase variation) the signal coherency during scanning on the target can only be attained for symmetric lobe patterns which results in a great limitation of the number of possible lobe patterns, in turn entailing a larger risk of detection than desired.
It is also known to distribute the radiated power spatially in several simultaneous time invariant lobes by means of a multilobe antenna thereby reducing the risk of detection. However, the relatively large angular value of the total sector angle of such a conventional multilobe radar antenna causes both the accuracy of measurment and the angular discrimination to become unsatisfactory.